Expandable pulse power spacecraft radiator

ABSTRACT

An expandable heat rejection system for radiating heat generated by a source of heat on a spacecraft or like vehicle is described and comprises a fluid heat exchange medium in operative heat exchange contact with the source for absorbing heat by evaporation of the liquid phase of the medium, a thin flexible wall structure having an inlet and an outlet and defining a volume expandable and collapsible between preselected limits and defining an inner condensation surface and an outer heat radiating surface, a multiplicity of capillary grooves on the condensation surface for promoting condensation of vaporous medium and for facilitating flow of condensate along the condensation surface toward the outlet, and a pump for circulating the medium through the system.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout payment of any royalty.

BACKGROUND OF THE INVENTION

The present invention relates generally to systems for radiating heatfrom spacecraft or the like, and more particularly to a variable,deployable heat exchange radiator for a spacecraft providing high peakto average heat rejection.

The profile of a power demand duty cycle of a space mission may includeperiods of high power draw over short times interspersed with muchlonger periods when power is needed at only a low level. The pulseduration, amplitude and frequency are determined by missionapplications. Conventional spacecraft radiators are sized to reject peakpower waste heat loads, and turned down to reject power loads during offpeak portions of a duty cycle. Conventional radiators are capable ofnear constant load thermal control over a range of nominally 10:1 peakto average heat loads for steady state heat rejection. However, for highpower applications requiring high peak to average heat rejectioncapability wherein system weight is constrained, conventional radiatordesigns are of limited utility.

The present invention is a variable volume and surface area deployableheat exchange radiator utilizing a two-phase heat exchange system totake advantage of the high boiling heat transfer rate of a heat exchangemedium at a heat source. The invention is characterized by a highcondensation heat transfer rate inside the radiator, low operating fluidmass due to the large latent heat of vaporization, and high radiatoreffectiveness due to near isothermal operation. The invention storessubstantially heat energy during a peak power load portion of the dutycycle for rejection of the stored heat during the off peak portion ofthe cycle. The invention is desirble for waste heat rejection where thepeak to average heat generation is large, i.e., greater than about 5:1,and can be sized for average duty cycle heat rejection and storage ofpeak power spikes for dissipation during off peak periods. The inventionmay be selectively structured for an operating temperature of 300° K.(low temperature electronic cooling regime), to about 1000° K. (spacepower system heat rejection regime).

For modest peak to average (e.g., to about 100:1) heat loads, a highsurface area to voluem rollout configuration is described. For higherpeak to average ratios (e.g., to about 10⁴ :1), an inflatable bag orbellows radiator structure having large volume to mass ratio isdescribed. The radiator is constructed of low mass, thin flexiblematerial which can be collapsed and stored, and which can be expandedreadily when high peak power heat loads are imposed. The bellowsstructure can take in large amounts of vapor during the peak (pulse)portion of the duty cycle, and reject the waste heat throughcondensation and radiation during the off peak portions of the dutycycle. The invention therefore provides a light weight radiator which iscompact and easily protected from micrometeroid impact except duringpeak expanded operation, and has minimum overboard contaminationproblems because the heat exchange medium is contained and recycled,rather than expelled overboard.

It is a principal object of the invention to provide a pulsed power heatrejection system for spacecraft or like vehicles.

It is a further object of the invention to provide a recycling heatrejection system having high peak to average heat rejection capability.

It is yet another object of the invention to provide a light weight,non-contaminating, expandable heat radiator having high peak to averageheat rejection capability.

These and other objects of the invention will become apparent as thedescription of representative embodiments proceeds.

SUMMARY OF THE INVENTION

In accordance with the foregoing principles and objects of theinvention, an expandable heat rejection system for radiating heatgenerated by a source of heat on a spacecraft or like vehicle isdescribed and comprises a fluid heat exchange medium in operative heatexchange contact with the source for absorbing heat by evaporation ofthe liquid phase of the medium, a thin flexible wall structure having aninlet and an outlet and defining a volume expandable and collapsiblebetween preselected limits and defining an inner condensation surfaceand an outer heat radiating surface, a multiplicity of capillary grooveson the condensation surface for promoting condensation of vaporousmedium and for facilitating flow of condensate along the condensationsurface toward the outlet, and a pump for circulating the medium throughthe system.

DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription of representative embodiments read in conjunction with theaccompanying drawings wherein:

FIG. 1 is a schematic of a control volume illustrative of an expandableradiator of the invention;

FIG. 2 illustrates a rollout expandable configuration for moderate levelpeak to average heat rejection;

FIG. 3 is an illustration of one expandable arm of the FIG. 2configuration;

FIG. 4 is a partial perspective view of an alternative expandable armstructure showing transverse capillary grooves on the condensingsurface;

FIG. 5 shows a representative structure for extending the arms of theFIG. 3 embodiment;

FIG. 6 shows a fan shaped constant radius expandable rolloutconfiguration; and

FIG. 7 shows an expandable bellows configuration for high level peak toaverage heat rejection.

DETAILED DESCRIPTION

Referring now to FIG. 1 of the drawings, shown therein is a schematic ofan expandable/collapsible control volume 10 for the expandable radiatorof the invention illustrating the geometry, mass flow, and heat transferprinciples in the operation of the invention. The invention comprises atwo-phase (liquid/vapor) heat transfer system utilizing thecharacteristic high latent heat of vaporization of a liquid heatexchange medium.

In the environs of outer space, a closed radiator system is required inorder to conserve and recirculate the heat exchange medium, and excessheat Q may therefore be dumped only by radiation, governed by thefamiliar expression,

    dQ/dt=εσA(T.sup.4 -T.sub.s.sup.4)

where ε is the emissivity of the radiating surface, σ is theStephan-Boltzmann constant, A is the radiating surface area, and T isthe absolute temperature of the radiating surface and T_(s) thebackground temperature. A high average (and uniform) temperature overthe radiating surface is therefore desired.

The background temperature of the near earth space environment varieswith orbital position, but in orbits of interest may be shown to averageabout 250° K. Electronic components and solid state circuitry haverepresentative mean time to failure of about 10⁴ hours at 475° K. whichtime reduces rapidly with increased temperature to about 10² hours atabout 575° K. Accordingly, for operating conditions for a radiator atheat rejection temperatures in the range of about 300°-400° K., water,ammonia, methanol and freons may be desirable as a heat exchange medium.For power system waste heat rejection in the high temperature (e.g.,400°-1000° K.) regime, liquid metals such as mercury, potassium, sodiumand lithium and sodium-potassium eutectics (NaK) are suitable for use asthe heat exchange medium.

Mass M of heat exchange medium flows into volume 10 (having variablevolume V and length L) at a rate dm/dt proportional to the heat transferrate and the enthalpy of evaporation of the heat exchange medium. Theliquid/vapor medium 11 within volume 10 may be assumed to be saturatedand transfers heat by convection and condensation to wall 12 definingvolume 10. Vapor inside volume 10 is isothermal at the saturationtemperature T_(sat), and the expansion/contraction of volume 10 isisobaric at the saturation pressure P_(sat), i.e., a constant externalpressure P_(o) is assumed to be reacting on volume 10 by reason of thestructure defining volume 10. Radiation heat transfer (shown by lines13) to space occurs from heated wall 12. The details of calculations forheat transfer utilizing the model presented in FIG. 1 is found in "LowTemperature Expandable Megawatt Pulse Power Radiator", by L. C. Chow, E.T. Mahefkey, and J. E. Yokajty, in Proceedings of the 1985 AIAAThermophysics Conference (June 19-21, 1985).

The expandable radiator of the invention may assume any of severalcontemplated expandable/collapsible structures depending on theanticipated peak to average heat loads to be encountered. FIGS. 2-5illustrate embodiments of the invention comprising high surface area tovolume rollout structures. The embodiments of FIGS. 2-5 may bepreferable for modest peak to average heat energy loads of from about5:1 to about 100:1. In radiator system 20 of FIG. 2, any convenientplurality of radially extending extendable heat exchange arms 21a-h maybe operatively attached to a source of heat 23 forming an integral partof a spacecraft or other vehicle 24 (represented by peripheral brokenline) enclosing the systems from which heat is to be extracted forradiation from system 20. With reference now specifically to FIG. 3,each arm 21 may comprise a sleeve 30 of flexible film material supportedby transverse structural elements 31, and having formed on the outeredges thereof fluid passageways 32,33 communicating respectively withinlet 34 and outlet 35 for passage of heat exchange medium into and outof sleeve 30. Passageways 32,33 communicate with each other across heatradiating surface area 30r of sleeve 30 wherein condensation of heatexchange medium and corresponding radiation of heat occurs in operationof the invention. Pressure sensor 36 for sensing pressure within sleeve31 may be operatively connected to inlet 34 and to any mechanismincluded in system 20 for extension of arms 21 as discussed below inrelation to FIG. 5. Pump 37 (suction, aspiration, or other suitabletype) circulates heat exchange medium through arms 21 from heat source23.

Referring now to FIG. 4, shown therein is a partial perspective view ofan alternative expandable arm structure including means defined in thecondensing surface for promoting the condensation of vaporous heatexchange medium and the flow of the condensate along the condensingsurface. Accordingly, a representative expandable wall structure for asleeve 40 may include a central inlet 40i for receiving vaporous heatexchange medium for condensation. A pair of liquid return channels 48,49flank the vapor condensation region of sleeve 40 substantially as shown.Liquid return channels 48,49 may be in the form of passageways similarto those described in relationship to the FIG. 3 embodiment, or maycontain wicking material to promote the the return flow of condensedheat exchange medium. Thin film walls 41,42 define the condensate regionand liquid return channels of sleeve 40, and comprise a thin flexiblefilm the respective outer surfaces 43,44 of which define the heatradiating surfaces of sleeve 40 for rejecting latent heat surrendered bycondensation of vaporous heat exchange medium within sleeve 40; innersurfaces 45,46 define the surfaces upon which condensation of the heatexchange medium occurs. A multiplicity of capillary tubes, channels orgrooves 47 (typically 0.005 inch grooves) on the inner surfaces 45,46 ofwalls 41,42 traverse sleeve 40 to maintain a liquid film inventory oninner surfaces 45, 46 and to assist in the flow of condensed heatexchange medium across sleeve 40 between passageways 48,49 as shown bythe arrows in FIG. 3 or 4. The condensate collected on the condensingsurfaces 45,46 of sleeve 40 (FIG. 4) or in area 30r (FIG. 3) is pumpedfrom the collecting passageways 48,49 (FIG. 4) or 32 (FIG. 3) by pump37, in the form of an aspiration pump or other suitable pumping means,in the recirculation of heat exchange medium to heat source 23.

In the operation of sleeve 40 for rejection of heat according to theteachings hereof, vapor flows into the condensation region of sleeve 40as indicated by the large open arrow of FIG. 4, condenses on surfaces45,46, and the condensate flows along grooves 47 normal to the vaporflow toward liquid return channels 48,49; liquid forms a fillet in thecorner regions of the film structure defining sleeve 40 due to the smallcapillary radii in those areas which provides some driving force for theliquid flow to liquid return channels 48,49. The principles governingvapor condensation and liquid flow at the condensing surfaces of sleeve40 may be extended in equivalent fashion to define flow of condensedheat exchange medium in other embodiments hereof.

Sleeve 30 comprises a light weight thin film material (selectedaccording to the design heat rejection capacity and selected heatexchange medium) of elastomer, metal, composite reinforced elastomer ormetal, prestressed metal foil, Kapton^(R), Kevlar^(R), plastic, rubber,metal/elastomer composite, fiber reinforced elastomer, or equivalentmaterial of thickness typically from about two to about ten mils. Arms21 may have an overall width of from about 1 to 10 meters and anextendable length of from about 10 to about 100 meters, depending on thedesign heat load. Each arm 21 may include two or more expandable sleeves(e.g., 30 or 40) formed or attached in side by side relationship andsimultaneously extendable in order to provide a compartmentalizedstructure having desirable heat rejection capacity. Further, heatconducting fins may be attached to or integral with each extenable armalong the sides thereof in order to provide additional radiating surfacearea for the rejection of heat.

A model rollout configuration of the type described above in relation toFIG. 4 was constructed in demonstration of the invention. Thedemonstration model was about one meter long by four inches wide havinga radiating surface of stainless steel foil about three mils thick witha heat exchange medium comprising water. The model functionedsatisfactorily in radiating heat according to the governing principlesof the invention.

Referring now to FIG. 5, shown therein is a representative mechanicalstructure for extending arm 21 for operating during peak heat loadpulses to vehicle 24. Each arm 21 may be structurally supported radiallyoutwardly of vehicle 24 and heat source 23 by a pair of spacedsubstantially rigid extendable booms 51,52. Arm 21 includes sleeve 50such as any of those described above, and may be deployed radiallyoutwardly or retracted using drum 53 supported between the distal endsof booms 51,52 and powered by motor 54. Extension means 55 for booms51,52 and motor 54 may be controlled electrically through control means56 and pressure sensor 57 with appropriate feedback represented by theconnecting lines. A pair of rollers 58 may be supported between booms51,52 as suggested in FIG. 5 to assist in deploying arm 21 and sleeve 50and to define the operating volume and radiating surface area of arm 21,and further to squeeze liquid and vapor from sleeve 50 during retractionof arm 21. In an alternate passive deployment and retraction embodimentof the invention, each expandable sleeve of the embodiments of FIGS. 2-5may define a prestressed, rollup or spring wound structure wherein thepressure of inflowing vapor overcomes the resiliency of the structure todeploy the sleeve. As condensation of vapor within the sleeve andaccompanying heat rejection by the sleeve surface occur, pressure andtemperature within the deployed sleeve reduce proportionally and theresiliency of the sleeve causes it to retract.

Referring now to FIG. 6, shown therein is an alternate configuration fordeploying a sleeve 60 similar in structure to sleeves 30,50 of the FIGS.2-5 embodiments. Sleeve 60 is supported on a structure including aplurality of spokes 61 and deployed or retracted in fan-like fashionfrom drum 63 or the like as boom 62 is rotated by centrally locatedmotor and control mechanism 66. Inlet 64 communicates with sleeve 60 andis operatively connected to suitable pumping and control equipment inmanner similar to the embodiment of FIG. 3. Spokes 61 may define liquidreturn conduits communicating with collector 65a and outlet 65 in thecirculation of heat exchange medium.

Referring now to FIG. 7, shown therein is another embodiment of theradiator of the invention comprising an expandable bellows particularlysuited for heat rejection at high peak to average heat load ratios up toabout 10⁴ :1. An extensible or inflatable bellows 70 having minimumsurface area to volume ratio (i.e., larger diameter to length ratio)comprises a thin film of the material comprising sleeves 30,40,50,60 ofthe embodiments of FIGS. 2-6 and is supported on an extensible largecompression helical spring 71 or other extensible support structure.Inlet 72 and outlet 73 of bellows 70 are operatively connected topumping and control equipment similar to that depicted in FIG. 3 for thecirculation of heat exchange medium. Wall 74 structure of bellows 70 hason the internal surface thereof capillary tubes, channels or grooveslengthwise of bellows 70 which define a condensation surface similar instructure to that depicted in FIG. 4. Bellows 70 may be deployed eitherby internal pressure of the system generated by evaporation of heatexchange medium at the heat source acting against the resiliency ofspring 71, or by mechanical means operatively connected to bellows 70for deployment thereof in manner similar to that depicted for sleeve 50of FIG. 5. By reason of the resiliency of spring 71, bellows 70 willextend to an equilibrium position depending on the internal pressure ofthe system in manner similar to the prestressed structure described asan alternative embodiment to the FIG. 4 structure; as vapor condenses onthe internal surfaces of bellows 70, spring 71 retracts bellows 70 andprovides a positive force to recirculate the condensed heat exchangemedium. In manner similar to that described in relationship to FIGS. 3and 4, an aspiration pump or similar pumping means may be operativelyconnected to outlet 73 for removing condensed heat exchange medium fromthe inner surface of wall 74, since capillary pressure within the liquidreturn channels on the inner surface of wall 74 may be insufficient toprovide a pressure head for return of condensed heat exchange medium. Apressure drop of less than one psi may ordinarily be needed at the pumpto maintain the liquid return even at very high pulse power heatrejection levels. Alternatively, the bellows may be provided with avariable diameter so that rdr/dx is constant, where r is the (variable)radius of the bellows and x a measure of extension of the length of thebellows, and, in conjunction with means to rotate the bellows (at a fewrpm), the resulting component of the centrifugal acceleration acting onthe condensate along the inner surface of the bellows assists in forcingthe condensate toward the outlet for recirculation. To reject peak heatloads of about 5 MW_(t), a three-meter diameter bellows 70 extendable toabout 20 meters and presenting a radiating surface area of about 180 m²and utilizing about 110 kg of heat exchange medium (e.g., water) may beappropriate.

An important feature of the expandable, deployable radiator of theinvention is that it can be compactly stowed and is subject to damagefrom micrometeoroid impact only when deployed for peak heat loadoperation (about 1/6 orbital time). Considering the near earthcumulative meteoroid flux, there is about a one percent chance for athree meter diameter by 100 meter long bellows to encounter a meteoroidof mass 10⁻⁵ gram or greater during one earth orbit; two encounters withmicrometeoroids of mass 10⁻⁷ gram or less may be expected. A holediameter caused by such an impact with a two mil surface structure isless than one millimeter and the resulting mass loss from a systemcontaining several hundred kilograms of heat exchange medium (water) maybe neglected. Freezing of condensed heat exchange medium in the vicinityof the hole may substantially reduce the effective hole size. If suchlosses are deemed unacceptable, a self sealing wall structure may beused, the structural details of which are outside the scope of theseteachings.

For each expandable configuration described in the foregoingrepresentative embodiments, a segmented or compartmentalized structuremay be devised to reduce significantly the probability of substantialfailure of an extended sleeve or bellows as a result of micrometeoroidimpact. Such structures may include two or more sleeves joined side byside as suggested above in relation to FIG. 4, or a bellows havingcompartments in the shape of cylindrical sections having a common inletalong the axis of the bellows, or other modifications as would occur tothe skilled artisan guided by these teachings, the same considered to bewithin the scope hereof.

The invention as hereinabove described therefore provides a novel heatrejection system for a spacecraft or similar vehicle. However, it isunderstood that structural materials, heat exchange medium and controlequipment, and size and operating temperature for the extendableradiator of the invention may be selected in consideration of specificmission peak heat loads, exposure times, system weight constraints andother parameters, as determinable by one skilled in the applicable artguided by these teachings. Therefore, all embodiments contemplatedhereunder which achieve the objects of the invention were not shown incomplete detail. Other embodiments may be developed without departingfrom the spirit of the invention or from the scope of the appendedclaims.

I claim:
 1. A heat rejection system for temporarily storing andradiating heat generated by a source of heat on a spacecraft or likevehicle, comprising:(a) a fluid heat exchange medium for contacting saidsource in operative heat exchange relationship with said source, saidmedium comprising a fluid material for absorbing heat by conversion ofsaid medium from the liquid phase of said medium to the vapor phase ofsaid medium; (b) a bellows defining a volume expandable and collapsiblebetween preselected limits, said bellows having an inlet and an outletand comprising a thin flexible wall having an inner surface defining acondensation surface for said medium and an outer surface defining aheat radiating surface; (c) means connected to said inlet for conductingsaid medium in said vapor phase from said source to said bellows; (d) amultiplicity of capillary grooves on said condensation surface of saidthin flexible wall for promoting condensation of said heat exchangemedium from said vapor phase to said liquid phase and for facilitatingflow by capillary action along said condensation surface generally in adirection from said inlet toward said outlet of the liquid condensateformed by said condensation; (e) means communicating with said outletfor conducting said liquid condensate from said outlet into said heatexchange relationship with said source; and (f) pumping means forcirculating said heat exchange medium through said system between saidsource and said bellows.
 2. The heat rejection system as recited inclaim 1 wherein said thin flexible wall of said bellows comprises a thinfilm of material selected from the group consisting of elastomer, metalfoil, composite reinforced elastomer, composite reinforced metal foil,prestressed metal foil, plastic, rubber, metal/elastomer composite, andfiber reinforced elastomer.
 3. The heat rejection system as recited inclaim 2 wherein said thin film is from about two mils to about ten milsin thickness.
 4. The heat rejection system as recited in claim 1 whereinsaid heat exchange medium comprises a material selected from the groupconsisting of water, ammonia, methanol, freon, mercury, potassium,sodium, lithium and sodium-potassium eutectic.
 5. The heat rejectionsystem as recited in claim 1 further comprising a spring operativelyconnected to said bellows for resiliently resisting expansion of saidbellows in response to the pressure within said system generated by saidconversion of said medium from said liquid phase to said vapor phase.